The purpose of this core is to provide histomorphometric analyses of undecalcified sections of animal and human samples. This technique forms an integral part of all five projects of the SCOR. Project 1 is testing the effects of ovariectomy and estrogen and intermittent PTH treatment on the osteoclast recruitment potential of mouse and rat bone marrow. Histomorphometry will be used to correlate indices of resorption and turnover in vivo with the number and activity of osteoclasts formed in vitro. In Project 2, we will continue to evaluate the rat model of osteoporosis, induced by two methods, surgical ovariectomy and gonadotropin releasing hormone inhibition. Core C will be required to follow and compare histomorphometric changes induced by these two types of ovarian failure and to correlate these with biochemical and bone mass changes, as assessed by Cores B and D. In addition, Project 2 will study the effects of PTH and estrogen treatment initiated at different times after bone loss has been induced. Core C will play an important role in evaluating the structural requirements for successful treatment and the cellular mechanisms involved. Both light and scanning electron microscopy will be used in these studies.